1. Field of the Invention
The present invention relates to a rectangular optical compensatory sheet, a process for the preparation of the sheet, a liquid crystal display provided with the compensatory sheet, and a color liquid crystal display provided with the compensatory sheet.
2. Description of Prior Art
As a display for electronic office system-devices such as a desk-top personal computer and a word processor, CRT (cathode ray tube) has been employed so far. Recently, a liquid crystal display (hereinafter referred to as LCD) has been widely employed instead of the CRT, because of its small thickness, light weight and low power consumption. LCD generally has a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. Most of LCD use a twisted nematic liquid crystal. Operational mode of LCD using the twisted nematic liquid crystal is roughly divided into a birefringence mode and an optical rotatory mode.
A super twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) utilizing the birefringence mode uses a super twisted nematic liquid crystal which shows a twisted angle of more than 90 degrees and having steep electro-optical characteristics. Such STN-LCD, therefore, has an advantage of giving display of a large area by driving in time-sharing mode. However, the STN-LCD has disadvantages such as slow response (such as several hundred milliseconds) and difficulty in giving satisfactory gradation on display, and therefore its display characteristics are relatively poor, as compared with display characteristics of a liquid crystal display using the known active-type elements (e.g., TFT-LCD and MIM-LCD).
In the TFT-LCD and MIM-LCD, a twisted nematic liquid crystal showing a twisted angle of 90 degrees and having positive birefringence is employed for displaying an image. This is called LCD of an optically rotary mode (i.e., TN-LCD). TN-LCD display mode shows quick response (such as several tens of milliseconds) and high display contrast, and easily gives a black-white display of high contrast. Hence, the optical rotatory mode has a number of advantages compared with the birefringence mode or other modes. However, TN-LCD has disadvantages that color or contrast on display varies depending upon viewing angle to a liquid crystal display, and its display characteristics are not comparable to the display characteristics of CRT.
In order to improve the viewing angle characteristics (i.e. to enlarge the viewable angle), arrangement of a phase difference film (optical compensatory sheet) between a pair of polarizing plates and TN liquid crystal cell has been proposed by Japanese Patent Provisional Publications No. 4(1992)-229828 and No. 4(1992)-258923.
The optical compensatory sheets proposed in these Publications give no optical effect when a liquid crystal display is seen from the direction vertical to a screen of the display because phase difference in the direction perpendicular to a surface of the liquid crystal display is almost zero. However, the optical compensatory sheet serves for compensation of phase difference (depending upon wavelengths of light) that occurs when the liquid crystal display is viewed from oblique directions. The phase difference results in unfavorable viewing angle characteristics such as coloring and disappearance of displayed image in the oblique directions.
It is known that the optical compensatory sheet is needed to have negative birefringence for compensating positive birefringence of the twisted nematic liquid crystal and an inclined optic axis.
Japanese Patent Provisional Publication No. 6(1994)-75116 and EP0576304 A1 disclose an optical compensatory sheet having the negative birefringence and inclined optic axis. In more detail, the disclosed sheet is prepared by stretching a polymer such as polycarbonate or polyester and has the directions of the main refractive indices which are inclined from the normal of the sheet. To prepare the above sheet by the stretching treatment, extremely complicated treatments are required. Therefore an optical compensatory sheet of a large surface area cannot be easily prepared according to the disclosed process.
Also known is an optical compensatory sheet comprising a liquid crystalline polymer. For instance, Japanese Patent Provisional Publications No. 3(1991)-9326 and No. 3(1991)-291601 disclose an optical compensatory sheet for LCD which is prepared by coating a solution of a polymer showing liquid crystal property on an orientation layer provided on a support film. However, the polymer showing liquid crystal property is not satisfactorily oriented on the orientation layer. Further, the polymer does not generally show negative birefringence. Hence, the resulting compensatory sheet scarcely enlarges the viewing angle from all directions.
Japanese Patent Provisional Publication No. 5(1993)-215921 discloses the use of a birefringence plate (optical compensatory sheet) comprising a support and a polymerizable lod-like compound which shows the liquid crystal property and positive birefringence. The birefringence plate is prepared by coating a solution of the lod-like compound on the support and curing the compound under heating. However, the cured layer dose not show the negative birefringence. Hence, the resulting compensatory sheet scarcely enlarges the viewing angle from all directions.
Thus, the above known optical compensatory sheets comprising a support film, an orientation layer of the polymer and a liquid crystalline compound layer cannot greatly enlarge the viewing angle from all directions.
EP 646829 A1 discloses an optical compensatory sheet greatly enlarging the viewing angle from all directions. The optical compensatory sheet has a representative structure comprising a transparent support, an orientation layer such as a rubbed polyvinyl alcohol layer thereon, and an optically anisotropic layer of a discotic liquid crystalline compound provided on the orientation layer. The use of the discotic liquid crystalline compound is stated to bring about enlargement of the viewing angle.
In EP 646829 A1, the optically anisotropic layer is formed by coating a coating liquid of a discotic liquid crystalline compound in methyl ethyl ketone on the orientation layer, and heating to form a discotic nematic phase.
The present inventor has studied this process in detail from the viewpoint of industrial application, for instance, to prepare an optical compensatory sheet of a large size, and found that when an optically anisotropic layer of a large size is prepared by the above process, the resultant optical compensatory sheet shows wide variation in its retardation within the effective surface area. Such local variation of retardation is naturally disadvantageous.